1. Field of the Invention
This invention relates to an oscillator circuit used in devices such as microcomputer.
2. Description of the Related Art
An oscillator circuit used in a semiconductor integrated circuit such as a microcomputer which uses a solid-state oscillator (a portion that generates oscillation of its resonant frequency when it receives voltage), such as crystal oscillator and ceramic oscillator has been widely known.
FIG. 3 is a simplified view of the oscillator circuit of prior arts. The oscillator circuit 100 has an inverter 101 made of a CMOS inverter as an amplifier circuit in a LSI. A clock signal CLK is outputted from the output terminal of the inverter circuit 101. The input terminal and the output terminal of the inverter circuit 101 are connected to a pair of outside terminals (outside connection terminals 102, 103). A feedback resistor 104, which gives bias to the inverter 101, is connected parallel to the inverter circuit 101 between the outside connection terminal 102 and the outside connection terminal 103.
Also, an outside oscillator 105 (a ceramic oscillator or a crystal oscillator) is connected between the outside connection terminal 102 and the outside connection terminal 103 outside of the LSI. Capacitor elements 106 and 107 are connected between the both terminals of the oscillator 105 and ground terminals GND. The electric capacity (F) of the capacitor elements 106 and 107 are designed to acquire a clock signal CLK of the desired frequency. This is the summary of the configuration of the oscillator circuit 100 of prior arts.
A certain amount of time is required for the oscillation of the oscillator circuit 100 to stabilize after the circuit starts oscillating. The duration of this time is called an oscillation stabilizing period hereinafter. The amplitude of the clock signal CLK is small and the signal does not have a high enough frequency component until the oscillation stabilizing period is passed. Therefore, the specification for the clock signal for operating a microcomputer can not be fulfilled until the oscillation stabilizing period is passed.
Therefore, the driving capacity of the inverter 101 is set high in order to shorten the oscillation stabilizing period in the oscillator circuit 100 of prior arts. That is, the size of the transistors (GW/GL), which make up the circuit, is set large for the inverter 101 to have low output impedance. Here, the GW means the channel width and the GL means the channel length of the transistor
Technologies mentioned above are disclosed in Japanese Patent Application Publication No. H10-224147.
However, while the oscillation stabilizing period is shortened, the consumption of the electric current increases when the size of the transistor making up the inverter 101 is large.
Therefore, this invention is directed to the oscillator circuit with a short oscillation stabilizing period and a low consumption of electric current.